1. Field of the Invention
The present invention relates to a noise filter and a noise-filter-incorporated amplifier circuit. Particularly, the present invention relates to a noise filter and a noise-filter-incorporated amplifier circuit in which radio frequency interference (RFI) removal characteristics is improved.
2. Background Art
An amplifier device is employed to perform impedance transformation and amplification in an electret condenser microphone (ECM). The amplifier device is formed, for example, of a junction field effect transistor (J-FET) or an amplifier integrated circuit device.
When the ECM is mounted, for example, on a mobile phone, a radio wave having a radio frequency of the mobile phone affects interconnections or associated parts for the ECM, and is detected as a noise of the ECM.
Various types of noise filters are used to improve RFI removal characteristics by preventing noise, such as a signal, from leaking or intruding through the interconnections. (This technology is described for instance in Japanese Patent Application Publication No. 2007-267168 and Japanese Translation of PCT International Application No. 2006-514497.)
FIGS. 5A and 5B are circuit diagrams showing conventional noise filters 510 and 511 to be connected to amplifier integrated circuit devices for impedance transformation, respectively. The noise filters 510 and 511 are, for example, low-pass filter (LPF) type electromagnetic interference (EMI) filters for preventing electromagnetic interference.
The noise filter 510 shown in FIG. 5A is obtained by connecting two capacitors C11 and C12 in parallel and by serially connecting a resistance R between one ends of the respective two capacitors C11 and C12 on their high voltage sides. The noise filter 510 is connected at its input terminal Vi′ to a power supply and at its output terminal Vo′ to an amplifier integrated circuit device (not shown).
This circuit is an LPF obtained by connecting the two capacitors C11 and C12 in a ladder manner and connecting the resistance between them. When the LPF circuit is connected to the amplifier integrated circuit device, incoming RF noise can be cut off by the LPF before reaching the amplifier integrated circuit device. Thereby, the RF noise which would otherwise affect the amplifier integrated circuit device can be drastically reduced, so that the RFI condition of the ECM can be improved.
The noise filter 511 shown in FIG. 5B is obtained by connecting diodes D1 and D2 instead of the capacitors C11 and C12. The noise filter 511 is an LPF which uses junction capacitors C21 and C22 of the diodes D1 and D2 as the capacitors C11 and C12 shown in FIG. 5A.
It is possible to integrate the LPF onto one chip with the amplifier circuit device, or with the J-FET or a p-channel metal oxide semiconductor field effect transistor (MOSFET) both of which are employed for impedance transformation like the amplifier integrated circuit device. In this case, the junction capacitors of the diodes D1 and D2 are generally used as shown in FIG. 5B from the viewpoint of the electrostatic discharge (ESD) and the manufacturing process.
FIG. 6 is a circuit diagram showing an example of a case where the noise filter 511 shown in FIG. 5B is connected to an amplifier integrated circuit device 550.
The amplifier integrated circuit device 550 is connected at its input terminal IN to one end of an ECM 560. The noise filter 511 is connected at its output terminal Vo′ on the high voltage side and at its output terminal (GND) on a low voltage side to the amplifier integrated circuit device 550 in parallel. Accordingly, the noise filter 511 and the amplifier integrated circuit device 550 can be integrated onto one chip.
In the noise filter 511 employing the diodes D1 and D2, a cut-off frequency fc is determined based on capacitance values of the junction capacitors C21 and C22 of the diodes D1 and D2 and on a resistance value of the resistance R. To satisfactorily improve the RFI condition, the cut-off frequency fc of the filter needs to be decreased. To decrease the cut-off frequency fc, either the capacitance values or the resistance value needs to be increased.
Increasing the capacitance values causes a problem of enlarging the size of the noise filter 511 or the chip size of a noise-filter incorporated amplifier integrated circuit device onto which the noise filter 511 and the amplifier integrated circuit device 550 are integrated.
Increasing the resistance value of the resistance R, on the other hand, causes a problem of output reduction (output loss) of the EMC 560. This is because the increase in the resistance value decreases a voltage outputted from the noise filter 511 to be applied to the amplifier integrated circuit device 550.